This invention relates to ignition devices for internal combustion engines, and more particularly to ignition timing control devices thereof comprising a microcomputer which is capable of adjusting the ignition timing to the optimum angle in accordance with the rpm of the engine.
Recently, it is becoming increasingly common in the automotive industry to utilize a microcomputer in the control of the ignition timing of an internal combustion engine. FIG. 1 shows a typical organization of such an ignition timing control device utilizing a microcomputer. A signal generator 1 comprises a rotor 1a and a pickup coil 1b; the rotor 1a, coupled to the crankshaft of the internal combustion engine to be rotated therewith, has a pattern of notches or projections (not shown in the figure) formed at the circumference thereof, so that the coil 1b generates a signal Sp consisting of a pattern of pulses which is repeated in each rotation of the crankshaft. The angular signal Sp from the signal generator 1 is inputted via an input circuit 2 to the microcomputer. The microcomputer itself comprises the following: CPU (central processing unit) 3 consisting of a microprocessor, an input timer 4 coupled to the input circuit 2, an output timer 5 coupled to the output circuit 11, an input port 6 which is coupled to various kinds of input circuits (not shown), a timer 7 within the microcomputer, a ROM (read-only memory) 8 in which various kinds of predetermined operation programs and data tables are stored, and a RAM (random access memory) 9 in which various kinds of temporary input and output data are stored; the above parts 3 through 9 of the microcomputer are coupled to each other by means of a bus 10. The output circuit 11 coupled to the output timer 5 consists of power transistors, etc., for turning on and off the current supplied to the ignition coil 12. The ignition coil 12 comprises a primary side coil 12a coupled to the output circuit 11, and a secondary side coil 12b across which ignition plugs (not shown) of the internal combustion engine are coupled. The current supplied to the primary side coil 12a is turned on and off by the output circuit 11 so as to induce a high voltage across the secondary side coil 12b, and hence across the gap of the ignition plugs.
The control of the ignition timing by means of the microcomputer of FIG. 1 is usually effected by means of a routine such as that shown in FIG. 2. In each ignition cycle, &he routine of FIG. 2 is initiated by the CPU 3 when an input signal interrupt is generated. Then, at step S3, the interval of time or period between successive pulses of the signal Sp, received from the signal generator 1 via the input circuit 2, is measured by the input timer 4; further, the ignition timing or the ignition angle before top dead center (BTDC) is determined on the basis of the period of the signal Sp which has just been measured; this determination of the ignition timing is effected by referring to a table stored in the ROM 8. The ignition timing thus determined is stored in the RAM 9. Next, at step S5, the CPU 3 effects an ignition timing prediction process on the basis of the current data, e.g., the ignition timing stored in the RAM 9. The routine thus ends at step S6. The ignition timing signal generated on the basis of this ignition timing prediction process is supplied to the output circuit 11; in response thereto, the output circuit 11 turns off the current supplied to the primary side coil 12a to induce a high voltage across the secondary side 12b at the precise point of time indicated by the ignition timing signal, thereby producing a spark across the gap of an ignition plug of the engine.
FIG. 3 shows the relation between the rpm of the engine (plotted along the abscissa) and the ignition timing (plotted along the ordinate). The ignition timing, as represented by degrees BTDC (before top dead center), is advanced as shown schematically by the solid curve A. (The relation such as that shown by the curve A is stored in the ROM 8 in the form of a table.) The intervals between the pulses of the signal Sp, as measured in angular degrees, are fixed according to the pattern of notches or projections formed on the circumference of the rotor 1a of the signal generator 1; thus, the intervals of time between the same pulses decrease as the rpm of the engine increases. Consequently, the intervals of time between the reference pulse shown near the bottom of Sp in FIG. 3 and the two pulses of Sp at the advance side near the top thereof become increasingly smaller as the rpm of the engine increases, as shown by the two dot-and-dash lines.
Thus, the above ignition timing control device has the following disadvantage: Let us assume that the interval of time or period between the two advance side pulses of Sp shown near the top in FIG. 3 is utilized in calculating the rpm of the engine and in determining the ignition timing at step S3 in FIG. 2. Then, in the high rpm region where the strip domain between the two dot-and-dash lines, which strip roughly corresponds to the ignition timing calculation time, comes near to the ignition timing, the proper ignition timing (represented schematically by the curve A) is lost before the determination of the period between the two advance side pulses of Sp and hence the calculation of the ignition timing based thereon (which are performed at step S3 in FIG. 2) are completed. Consequently, the angular interval between the reference pulse (shown near the bottom in FIG. 3) and the advance side pulses (shown near the top) of the signal Sp must be made sufficiently great to ensure that the calculation time remains even if the rpm of the engine increases to or near its maximum. Consequently, the interval of time between the time at which the calculation of the ignition timing is effected and the time at which the ignition is actually effected becomes longer in the low rpm region. If the rotational speed of the engine varies during this interval of time between calculation and ignition, a deviation of the ignition timing from the optimum angle may result. That is, varying rotational speed of the engine may result in undesirable deviations of the ignition timing.